Arenaviruses cause severe viral hemorrhagic fevers in humans. Five arenaviruses have been classified as NIAID Category A pathogens due to their potential for misuse and the severity of the disease they cause: lymphocytic choriomeningitis (LCM), Lassa, Guanarito, Machupo, and Junin viruses. LCM and Lassa viruses are Old World arenaviruses that use the cellular receptor ?-dystroglycan to enter cells. Guanarito, Machupo, and Junin viruses, all New World hemorrhagic fever arenaviruses, are thought to use a common receptor, but this receptor has not been identified. In preliminary data, we demonstrate a high affinity association between transferrin receptor 1 (Tfr1) and the entry glycoprotein (GP) of Machupo virus. Human Tfr1 markedly increased the efficiency with which Machupo or Junin pseudoviruses, but not Lassa pseudovirus, infected a weakly permissive hamster cell line. Entry into human cell lines of Machupo pseudovirus, but not Lassa pseudovirus, was abolished by anti-human Tfr1 antibody, but not by a control antibody. Our data indicate that Tfr1 is an obligate cellular receptor for New World hemorrhagic fever arenaviruses. Our aims are to: (1) fully characterize the role of Tfr1 and Tfr2 in the entry processes of an extensive panel of arenaviruses, (2) identify Tfr1 and GP determinants of their high affinity association, (3) characterize the role of Tfr1 species variation in arenaviral entry, (4) describe the effect of soluble Tfr1, and of various anti-Tfr1 antibodies on entry, (5) describe the role of iron and of Tfr1- associated proteins in arenaviral entry, (6) collaborate to solve the structure of GP bound to Tfr1. These studies will shed light on why some, but not all arenaviruses cause hemorrhagic fevers in humans, and will help assess the risk posed by those that have not infected humans. They will also contribute to the development of protein and small-molecule therapeutics for South American hemorrhagic fevers.